810 
3 - DESCRIPTION OF THE INSTRUMENT 
The optical front end of the radiometer is composed of a thermal detector (thermopile) located in the conjugate 
plane of an objective lens with respect to a condenser, the latter being located in the focal plane of the 
objective, as shown in Figure 1. This so-called Köhler arrangement has several advantages: 
- The energy flux incident upon the detector is homogenized; this eliminates possible hot spots which could 
harm the detector or produce erroneous measurements, 
- Vignetting is prevented, 
- The object field of view can be modified without changing the image field of view which allows the 
instrument to be developed in an evolutive prospect. 
Figure 1. Optical front end of the instrument 
(i) The detector characteristics given by the constructor are summarized in Table 1. 
Active area 
0.6 x 0.6 mm 
Number of junctions 
40 
Resistor 
60 kli 
Voltage noise 
31.3 nV/Hz 1 / 2 
Responsivity 
120 V/W 
Responsivity temperature coefficient 
-0.15%/K 
Noise Equivalent Power (NEP) 
0.26nW/Hz 1/2 
D* 
2.3 10 8 cm/W/Hz 1 / 2 
Time constant 
12 ms 
Field of view 
OO 
o 
o 
Germanium window with "non-reflective" coating 
8 - 14 pm 
Table 1. Detector characteristics. 
(ii) The condenser is a "best-shaped" meniscus of very short focal length allowing geometrical aberrations to 
be minimized. It is made of germanium and carries a non reflective coating. 
(iii) The objective lens is a germanium convex-plane lens with a non-reflective coating. It has standard 
dimensions. There is one objective for each field of view. The main characteristics of the lenses are shown in 
Table 2.